1. Field of the Invention
The present invention relates to a transmitter for a plurality of carriers of a broad-band transmission spectrum.
2. Description of the Prior Art
Transmitters of the afore-mentioned type are preferably utilized in radio stations within mobile radio networks in which a transmission of information to further radio stations occurs in a plurality of carriers with the aid of radio-frequency signals. These radio stations are base stations which are stationary and which carry out information transmission to mobile stations, for example. In a transmitter, input signals are amplified into more powerful output signals and are converted into a corresponding transmitting frequency.
If a broad-band transmitting spectrum with a plurality of carriers is amplified, linearity problems of amplification arise. In order to keep the power efficiency of the power amplifier within the transmitter high, the latter is usually operated in AB, B or C mode. However, these modes are distinguished by highly non-linear behavior which results in a poor intermodulation suppression. Without suitable linearizing processes, such power amplifiers in transmitters with a plurality of carriers are not suitable when a minimal intermodulation suppression is required.
It is generally known to amplify each carrier individually and, thus, to maintain the linearity. The transmitter is then not broad-band, however, and must be correspondingly frequently made available in the radio station. In the carrier-specific amplification, a merging of the carriers occurs at the amplifier output by means of hybrids, filters or antennas. The individual amplification branches are shielded from each other by insulators. Harmonic distortions are suppressed by transmitting filters.
K. J. Parson, P. B. Kenington, “The efficiency of a feed-forward amplifier with delay loss”, IEEE Trans. on Vehicular Techn., Vol. 43, May 1994, pages 407-412 teaches to perform the linearizing of a broad-band transmitting spectrum for a plurality of carriers with the aid of a “feed forward” method. A loop of a main amplifier and an error amplifier is therein constructed. The error amplifier amplifies only the intermodulation distortions of the main amplifier, wherein such distortions are subsequently subtracted from the output signal of the main amplifier. The disadvantages of this method are a) the necessary analog tuning precision in rate and phase at the summation points, and b) a lower power efficiency resulting from the utilization of two amplifiers, from the loss of couplers and from the loss of delay units. At the moment, however, this method only approximately achieves the intermodulation suppressions required by the GSM (Global System for Mobile Communication) mobile radio system in a double loop execution.